The present invention relates to a new and improved construction of an electrical drive device comprising a stator arrangement of the claw or dog pole type, surrounding a rotor compartment or chamber and generating therein at least one magnetic field having at least a rotary field component.
It is well known in the art that basically there are available three types of electric motors which do not employ collector- and slip rings, and the stator of which is supplied with single-phase or polyphase alternating-current.
The first type is the so-called permanent magnet-synchronous motor, the rotor of which comprises one or a number of permanent magnets. These permanent magnets, especially in the case of small motors, are usually fixedly mounted in a cylindrical rotor and are flush with its smooth outer surface. The stator is then advantageously of the so-called claw pole type, resulting in a particularly simple, inexpensive and small construction. These synchronous motors, can possess quite a high mechanical torque as long as the rotor rotates at the synchronous rotational speed, but however the mechanical torque becomes practically equal to null when a braking moment, which is greater than that capable of being delivered by the motor, reduces the rotational speed of the rotor.
The second type is the so-called hysteresis synchronous motor, the rotor of which is formed of a material possessing hysteresis, i.e. a magnetic material characterized by quite high remanence, but low coercive force and furthermore possesses as high as possible specific resistance. These synchronous motors can deliver a pre-determined mechanical torque and rotate at the synchronous rotational speed as long as the braking moment does not exceed this pre-determined torque. If the braking moment exceeds this torque which is characteristic for the motor, then the rotor begins to rotate less rapidly and can even come to standstill, and the torque delivered by the motor always is equal to the relative pre-determined torque. If the braking torque decreases with the rotational speed, for instance, in the presence of liquid- or gas friction, then the motor begins to rotate at a rotational speed where the braking torque is exactly equal to the characteristic torque of the motor. If the rotor is braked until coming to standstill and the braking torque is converted into a reaction moment, which is automatically equal to the torque of the motor, then the latter always possesses its characteristic value, and the rotor again begins to revolve when the braking action only develops a torque which is less than that delivered by the motor.
Finally, the third type is the so-called induction current-asynchronous motor which comprises a rotor which at least in part is formed of a conductive material. There are distinguished between asynchronous motors of appreciable output, enclosing a magnetic rotor which is equipped with conductor rods in a squirrel cage arrangement and asynchronous motors of lower output which enclose a rotor which is only composed of non-magnetic material of good current-conducting properties, these motors also being designated as eddy current motors.
In accordance with these three motor types, there are also known to the art three types of magnetic or electromagnetic couplings, wherein the driving or driven part comprises one or a number of permanent magnets which rotate and produce a magnetic rotary field. The other part is always a permanent magnet-, a hysteresis-, or an eddy current-part and is actuated by this rotary field in exactly the same manner as the rotor in a motor of the corresponding type.
These three types of magnetic couplings have been particularly described and examined in an article authored by Paul Gernhardt appearing in "DEW-Technische Berichte", 2. Band, 1962, Heft No. 4, pages 153-159. This article contains very detailed information which need not here be repeated.
Of these three motor types the hysteresis motor possesses properties which are particularly interesting from a number of different standpoints. There is however one point with regard to which it possesses an appreciable drawback in relation to motors equipped with permanent magnets. The hysteresis motor only operates really satisfactorily if the magnetic field to which the rotor is exposed possesses a true uniform rotary field, i.e. a rotary field like that in a magnetic coupling where the field is generated by a magnet which rotates. If there is available, for instance, a three-phase stator arrangement, then it is easy to produce a true magnetic rotary field. However, if there is only available a single-phase alternating-current power supply, then oftentimes it is desirable to be able to be satisfied with a simple alternating field or at best an elliptical field which consists of a primary magnetic field and an appreciably weaker magnetic field which is phase-shifted by 90.degree.. In this case, the hysteresis motor produces really unsatisfactory results, since in a simple alternating field, rendering possible the operation of a permanent magnet motor, the hysteresis motor is not capable of operating. On the other hand, if there is available, especially in the case of smaller motors, two windings, through one of which there can flow a current which, related to the current flowing through the other winding, is phase-shifted, it is theoretically possible to produce a relatively uniform rotary field. However, for this purpose, the effect of both windings must be combined at one identical axial section of the rotor compartment or chamber, leading to considerable constructional difficulties and thereby annihilating the aforementioned constructional advantages of the stator of the claw or dog pole type. In the case of small permanent magnet motors it is usually preferred to have avilable two axial adjacently arranged stator arrangements, in that a rotor is equipped with a permanent magnet which extends centrally into both stators. Then each of the stators delivers a simple alternating field, of which one is merely phase-shifted in time with respect to the other. The rotor is then driven by one stator at one moment in time and by the other stator during another moment in time, and the field which is obtained, while indeed in the nature of a certain type of rotary field, however is more correctly describable as "wobbling". Usually one of the stators is directly powered with single-phase alternating-current from the power supply network, whereas the other stator is supplied from such power supply network by a capacitor connected in series therewith. Depending upon whether there is supplied the one or the other winding by means of the capacitor, the rotor rotates in the one or the other direction. Yet, it will be observed however that such type "wobbling" rotary field in the case of a hysteresis motor does not afford any advantages in contrast to a simple alternating field.
If it is desired to obtain for a small motor the particularly interesting properties of a hysteresis motor, then it is necessary to provide a relatively complicated construction of the stator, and if it is desired to maintain a simple stator construction, then, there must be used a permanent magnet motor and there must be dispensed with the properties of the hysteresis motor.